The invention relates to a portable radio transmitter especially suited as a remote for a motor vehicle. Electronic keys are being used increasingly instead of mechanical keys for modern automobiles as device which authorize access and/or use. In the special case of remotes, the encoded information authorizing access and/or use is transmitted to the vehicle. Conventional remotes use an oscillator as a voltage signal source acting in conjunction with the antenna. However, this can result in relatively large scatter of the transmitting power of the remote, typically up to 6 dB and, with temperature drift, up to approximately 9 dB, so that a one-third reduction of the original desired range may occur.
A mobile radio transmitter, in other words, a handheld transmitter or cell phone, is known from German Patent Document DE 43 39 526 A1. In this handheld radio, the AC voltage signal generated by a voltage signal generator, consisting of a high-frequency carrier signal with a time-compressed modulated speech signal, is supplied to a power-controllable HF driver amplifier. The output signal is supplied to an HF power end stage with constant power amplification, whose output signal, in turn, is supplied through an HF output filter to an antenna. Additionally, the output signal from the HF driver amplifier is supplied through an isolating amplifier to an HF rectifier stage. The isolating amplifier delivers an output signal proportional to the HF power. In the rectifier stage, the amplifier output signal is guided through a coupling capacitor to the cathode side of an HF rectifier diode, which is grounded, on the anode side, through another capacitor, and is also supplied through an HF choke with a reference voltage that functions as the DC bias. A parallel, identically structured diode branch provides DC bias compensation in the rectifier stage, whose output voltage on the cathode side is subtracted from the voltage of the rectifier diode in a summing amplifier. Thus the summing amplifier delivers only the rectified HF voltage on the output side. This summing amplifier output signal is supplied to the inverting input of an operational amplifier. The non-inverting input of the operational amplifier is supplied with a power setpoint voltage representing the desired output power. The output signal of the amplifier serves as the control voltage for adjusting the power of the HF driver amplifier.
The present invention provides a portable remote radio transmitter, which has a simple compact design, and which has no undesirable fluctuations in the radiated transmitting power. The radio transmitter includes an oscillator as the voltage signal generator with the power-regulating amplifier stage containing at least one bipolar transistor having its base electrode connected to the oscillator. A current-adjusting element is looped in the collector-emitter current path and is controlled by an adjustment signal from the operational amplifier. The rectifier stage has diodes connected in parallel, in opposite directions, and one resistor and one capacitor wired in parallel to one of the diode at the corresponding operational amplifier input.
With this arrangement, transmitting power can be regulated so that undesired transmitting power fluctuations can be prevented. If the power setpoint voltage is adjustable, the radio transmitting power can be varied, as needed. Because of its simple design, the remote is especially suitable for an automobile or other vehicle.
According to another feature of the present invention, the power setting voltage is supplied to the operational amplifier by a temperature-compensating diode, which compensates for any temperature drift in the rectifier stage of the diode. For this purpose, the compensating diode is advantageously positioned sufficiently close to the diode of the rectifier stage so that both diodes are subject to the same temperature influences. In particular, when an integrated circuit design is used for the radio transmitter, both diodes are on a common chip surface.
According to one aspect of the present invention, the amplifier stage consists of a pair of bipolar transistors each having a base electrode connected to one of two signal outputs of the oscillator. The current-setting element, controlled by the operational amplifier, consists of a transistor in a current-level circuit formed of two transistors, by which the current strength in the collector-emitter current path of the two amplifier transistors, and hence their amplification and the power they deliver, can be varied. It is possible to combine the four required transistors together with the oscillator into an integrated circuit.
When using a magnetic radio-transmitting antenna, the feedback voltage signal is obtained through a feedback antenna.
In another aspect, a power setpoint voltage adjusting unit sets several different power setpoint voltage values so that, depending on the requirement, different transmitting powers of the radio transmitter can be selected.
ASK or AM modulation can be provided by a corresponding ASK or AM modulation unit which generates a corresponding modulation signal for the power setpoint voltage.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.